One area the property mogul won’t touch again: property. “These are my heartfelt words: I do not want to earn money in such an exhausting way anymore,” he said.

Mr. Yan is one of a growing number of developers who see no end to the pain in Chinese real estate and are looking to get out. Other developers are branching off into everything from banking to cosmetic surgery to women’s soccer.

That skepticism suggests China’s property market—an important driver of growth in the world’s No. 2 economy—isn’t likely to come to the rescue as the country’s growth slows. Some even suggest a long-term de-emphasis on property as China’s birthrates slow and the population ages.

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According to Moody’s Analytics, housing and its related industries contributed 18% of the country’s gross domestic product in 2014, down from 23% in 2013.

“China’s property market has reached an alarming turning point,” said Ma Guangyuan, a Beijing-based independent economist. He added, “the property sector has turned from being China’s economic growth engine into its burden.”

In almost all but the richest places such as Beijing, Shanghai, Shenzhen and some provincial capitals, China’s cities are plagued with weak demand and a massive glut of unsold apartments. While housing sales are up 7.9% by volume in the first 11 months this year from a year ago, construction starts have been declining at double-digit levels. Growth in investment in residential property slowed to 0.7% growth in the first 11 months of this year, down from 9.2% for last year.

The latest venture by Lek Yuan Group is art, including a Hong Kong modern-art gallery focused on Chinese painters. Lek Yuan Group

In August, Hangzhou-based Lander Real Estate Co. changed its name to Lander Sports Development Co. and sold some of its real-estate assets to a shareholder so that it could focus on developing its new sports business, which includes a recent plan to invest in Hangzhou’s women’s soccer team. Lander reported a 37% decline in net profit in 2014, following a 7.3% increase in 2013.

Shanghai-listed Deluxe Family Co. is looking to invest in robotics, near-space flying vehicles and graphene, a thin material for use in the manufacture of mobile phones. The residential property developer, which has a few remaining property projects in the city of Suzhou, changed its business license to become a firm focused on investment management and consulting. Deluxe Family said the moves were made to increase new profit lines.

In May, real-estate firm Shanghai Duolun Industry changed its name to P2P Financial Information Service Co. to diversify into Internet finance and consultancy services, driving its shares sharply higher.

Major players are staying in property but broadening their revenue sources. Evergrande Real Estate Group Ltd., which has recently spent $2.1 billion buying uncompleted property projects, has delved over the past two years into mineral water, dairy, grains and oil. It also runs a cosmetic surgery hospital in Tianjin, though in March it scrapped its solar energy venture because “timing wasn’t right.” Evergrande didn’t respond to requests for comment.

In the first half of 2015, Evergrande recorded revenue of 1.6 billion yuan ($247.8 million) from businesses apart from property development, investment and management. That marked only 2% of its revenue but was double the amount from the same period a year earlier.

Mr. Yan is chairman of Lek Yuan Group, a property developer in the eastern Chinese city of Nanjing. The former newspaper reporter decided to start his own property developer in the early 1990s with the help of contacts he had accrued. Over the years, his firm built thousands of homes across four Chinese cities.

The business prospered, but Mr. Yan said the complications and red tap—a project takes about 200 stamps of approval—took their toll. “People see developers leading glamorous lives and getting interviewed by reporters, but there are also times where you have to worm your way through a dog hole,” he said. “It’s a painful and bitter process.”

China’s efforts to tame an earlier housing-price surge added to the company’s debt load and first turned Mr. Yan’s thoughts toward diversification. Yet another boom in 2012 brought in new competitors who drove up land prices. “The barriers to entry are too low,” Mr. Yan said. “Nanjing has more than 1,000 property developers. That’s absurd. The number should not exceed 50.”

As the current slump worsened, Mr. Yan hastened efforts to diversify. First he turned to the water-skimming plane, or what is called a ground-effect vehicle, which flies over water the way a seagull does. But he discovered China’s military authorities, which control airspace in the country, had no plans to loosen restrictions soon, so he sold the operations for one yuan.

His latest venture is art, including a Hong Kong modern-art gallery focused on Chinese painters and an annual art fair in Nanjing. “I wanted to look for a positive, public, transparent and relaxing business,” said Mr. Yan.

He said he held meetings with his staff explaining his analysis of China’s property market. Some decided to stay with his firm, he said, and now appraise art instead of property. “The value of art will only appreciate,” he said.